The paranasal sinuses (latin: sinus paranasales) are four bilateral air-filled spaces within bones of the skull surrounding the nasal cavity. Four bones of the skull each accommodates a pair of paranasal sinuses that are named according to the bone in which they are located. The four sinuses are:
- maxillary sinus,
- frontal sinus,
- sphenoidal sinus,
- ethmoid sinus (known as ethmoidal air cells).
All paranasal sinuses open into the nasal cavity and they are considered to belong to the respiratory system. All sinuses are lined with the respiratory epithelium.
Although functions of the paranasal sinuses are not completely clear, they are believed to humidify the inhaled air and contribute in voice resonance. The paranasal sinuses also reduce the weight of the skull.